Known in the art is an internal combustion engine having arranged in an engine exhaust passage an NOx storing catalyst which stores NOx contained in exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and releases the stored NOx when the air-fuel ratio of the inflowing exhaust gas becomes the stoichiometric air-fuel ratio or rich. In this internal combustion engine, the NOx produced when burning fuel under a lean air-fuel ratio is stored in the NOx storing catalyst. On the other hand, when the NOx storing capability of the NOx storing catalyst approaches saturation, the air-fuel ratio of the exhaust gas is temporarily made rich and thereby the NOx is released from the NOx storing catalyst and reduced.
However, the fuel and lubrication oil contain sulfur. Therefore, the exhaust gas contains SOx. This SOx is stored in the NOx storing catalyst along with the NOx. However, this SOx is not released from the NOx storing catalyst just by making the air-fuel ratio of the exhaust gas rich. Therefore, the amount of SOx stored in the NOx storing catalyst gradually increases. As a result, the amount of NOx which can be stored ends up gradually decreasing.
Known in the art therefore is an internal combustion engine having an SOx absorbent arranged in the engine exhaust passage upstream of the NOx storing catalyst so as to prevent SOx from being sent to the NOx storing catalyst (see Japanese Unexamined Patent Publication (Kokai) No. 2000-145436). In this internal combustion engine, the SOx contained in the exhaust gas is absorbed by the SOx absorbent. Therefore, the SOx is prevented from flowing into the NOx storing catalyst. As a result, the storing capability of the NOx can be prevented from dropping due to storage of SOx.
When using such an SOx absorbent, however, if the SOx absorption capability of the SOx absorbent ends up being saturated, the SOx will end up flowing into the NOx storing catalyst. With this SOx absorbent, however, if raising the temperature of the SOx absorbent and making the air-fuel ratio of the exhaust gas flowing into the SOx absorbent rich, it is possible to release the absorbed SOx from the SOx absorbent and therefore possible to restore the SOx absorbent. However, releasing SOx from the SOx absorbent in this way, the released SOx will end up being stored in the NOx storing catalyst. Therefore, in this internal combustion engine, a bypass passage bypassing the NOx storing catalyst is provided. When releasing SOx from the SOx absorbent, the released SOx is exhausted into the atmosphere through the bypass passage.
In the above SOx absorbent, by raising the temperature of the SOx absorbent and making the air-fuel ratio of the exhaust gas flowing into the SOx absorbent rich in this way, it is possible to release the SOx from the NOx absorbent. However, in this way, the SOx is only released from the SOx absorbent little by little. Therefore, for release of all of the absorbed SOx from the SOx absorbent, the air-fuel ratio must be made rich for a long time and therefore there is the problem that a large amount of fuel or a reducing agent becomes necessary. Further, the SOx released from the SOx absorbent is exhausted into the atmosphere. This is also not preferable.
In this way, when using an SOx absorbent, if the release of SOx is not controlled, limits end up arising in the SOx absorption capability. Therefore, when using an SOx absorbent, the release of SOx must be controlled. However, if the release of SOx is controlled, that is, so long as SOx is made to be released from the SOx absorbent, the above problem arises.